The present invention relates to optical switch arrays and, more particularly, to an integrated optical switch array in which arbitrary combinations of the inputs and outputs are explicitly addressable.
Integrated optical switches are well-known. For an early review of the art, see Lars Thylen, "Integrated optics in LiNbO.sub.3 : recent developments in devices for telecommunications", Journal of Lightwave Technology vol. 6 no. 6 (June 1988), pp. 847-861. Waveguides are created in a lithium niobate substrate by processing the substrate locally to increase the index of refraction. For example, the index of refraction of lithium niobate may be increased locally by diffusing titanium into the substrate. To divert light from one waveguide to another, the waveguides are coupled by local optoelectrical manipulation of their indices of refraction. Well-known examples of optoelectrical switches include directional couplers, BOA couplers, digital optical switches and x-switches. Depending on the voltage applied to such a switch, light is thus partly or completely diverted from an input waveguide to an output waveguide.
By appropriately combining waveguides and switches, a switch array is formed to switch light from a plurality of input waveguides among a plurality of output waveguides. A variety of switch geometries are known. FIG. 1A is a conceptual illustration of a switch of one such geometry: crossbar geometry. A set of input waveguides 10 crosses a set of output waveguides 12. At the crossing points, the waveguides are coupled by 2.times.2 switches 14. For simplicity, only three input waveguides 10 and three output waveguides 12 are shown in FIG. 1A. Typically the numbers of input waveguides 10 and output waveguides 12 are equal powers of 2, up to a practical maximum of 32.
FIG. 1B shows, schematically, the actual layout of the switch array of FIG. 1A. Switches 14 are shown as directional couplers, in which parallel segments of the waveguides are flanked by electrodes (not shown) to which the coupling voltages are applied. Note that input waveguide 10a leads directly into output waveguide 12a, that input waveguide 10b leads directly into output waveguide 12b, and that input waveguide 10c leads directly into output waveguide 12c. To allow arbitrary coupling of inputs to outputs, two auxiliary waveguides 11a and 11b are provided. Waveguides 10a-12a and 10b-12b are coupled in switch 14a. Waveguides 10b-12b and 10c-12c are coupled in switches 14b and 14c. Waveguides 10c-12c and 11a are coupled in switches 14d, 14e and 14f. Waveguides 11a and 11b are coupled in switches 14g and 14h. Note that switches 14d and 14g actually are 1.times.2 switches, that switches 14f and 14h actually are 2.times.1 switches, and that there is no switch corresponding to the lowermost 2.times.2 switch 14 of FIG. 1A. (A 1.times.2 switch is a 2.times.2 switch with one input deactivated; a 2.times.1 switch is a 2.times.2 switch with one output deactivated.) Switch arrays based on geometries such as the crossbar geometry of FIGS. 1A and 1B can be used to divert input signals to output channels arbitrarily. Signals from any input channels can be directed to any output channel, and even to multiple output channels, in broadcast and multicast transmission modes. One drawback of known optical switch array configurations is that it is difficult to determine how to configure the switch to achieve a desired coupling of input and output channels. In general, in order to configure a switch array as desired, on the order of N! switch combinations may have to be tested computationally to find the desired combination. In large switch arrays, the time required for this computation is the rate limiting factor in switch array speed.
In the days before integrated optics, Fulenwider, in U.S. Pat. No. 3,871,743, described an optical switch array in which input optical fibers are coupled explicitly to output optical fibers. Each input optical fiber is coupled to each output optical fiber by only two "input ports". In such a switch geometry, the amount of time needed to decide which "input ports" to activate to achieve arbitrary coupling of inputs to outputs is linear in the number of coupled channels. Unfortunately, the particular embodiment described by Fulenwider is not well-suited to fabrication as an integrated optical device.
There is thus a widely recognized need for, and it would be highly advantageous to have, an integrated optical switch array, for arbitrary coupling of input channels to output channels, in which the computational burden is linear in the number of coupled channels.